It means to be unusually nice to someone because you want something from them, like a small favour, but you haven't asked them yet.

to "butter up" someone means to treat them nicely, usually because you want something from them. It is similar to the phrasal verbs "to kiss up" or "brown nosing". To use this in a sentence, the object of the sentence can be placed after 'butter up', or in between 'butter' and 'up'. For example, when a student is being really nice to a teacher to get a better grade, you can say, "That student is buttering up the teacher" or "That student is buttering the teacher up" This comes from cooking. When you put more butter on some bread, or butter it up, you are covering it with yumminess. haha. So, it the people sense, the person you are flattering is the bread, and the butter is the flattery.

To praise or flatter someone excessively

Everyone else has given excellent explanations of 'butter up', which just leaves one thing for me to do - correct the horrible mistake in your question. If your English level is good enough for you to be worrying about idioms, you really shouldn't be making mistakes with questions with the present tense of 'to be'. What IS the meaning of 'Butter up' ? Do pay attention to basic grammar. When you're using English in your studies, work or daily life, nobody will notice whether you know the meaning of 'butter up' or not. They will notice if you can't construct a simple question correctly.

As Kevin has said, it's excessive flattery, but specifically in order to get something out of that person later. For example, you say nice things to a person and treat them well, maybe buy that person dinner, so they'll agree to your plans... which you'll tell them about later. This is a little off-topic, but "to butter (someone) up" reminds me of the Slovak phrase, "mazať med okolo úst", which has the same meaning and literally translates as "to spread honey around (the other person's) mouth." Mmmm. Is there an idiom in Indonesian which has the same meaning?